For many applications, it is desirable to employ a semiconductor laser that operates in the 2-5 micrometer (.mu.m) range and provides a high output power. Typically, a semiconductor laser may be constructed of layers formed on a binary substrate, such as GaSb, InP, InAs or GaAs. While other substrate materials exist, these materials are commonly employed for substrates in the art. Some applications have employed semiconductor lasers having ternary compounds for the waveguide and active region, while other applications have employed quaternary compounds.
An important aspect of forming these layers in semiconductor lasers of the prior art is matching of a lattice constant of the layers of waveguide and active regions with the lattice constant of the substrate, which is normally a fixed value. Lattice matching avoids dislocations, or misfits, in crystal epitaxial layers.
For ternary compounds, the lattice constant depends on the composition, and if quantum well ternary material is lattice-matched with a waveguide region at a certain composition, the deviation from this composition leads to tensile or compressive strain of the ternary material. As is known in the art, a series of calculations derived by Matthews and Blakeslee may be used to calculate misfit dislocation concentration and critical strain for a pair of parallel crystal materials with different lattice constants that are placed in contact. These calculations are described in, for example, J. W. Matthews, J. Vac. Sci. Technol. 12, pages 126-133, 1975; J. W. Matthews, Dislocations in Solids, ed. F. R. N. Nabarro (Elsevier, New York, 1979), v.2, p461; and J. W. Matthews and A. E. Blakeslee, J. Crystal Growth, vol. 27, pages 118-125, 1974, and these articles are incorporated herein by reference for their teachings of misfit dislocation and critical strain calculation.
One III-V material is employed for the waveguide region and another is employed for the active region. As is known in the art, the active region may be a quantum well (QW) or multiple quantum wells (MQW). From the calculations described by Matthews and Blakeslee, one can find the critical value of the product (f*h).sub.crit, where h is the QW thickness, and f is the lattice mismatch between the QW and the adjacent waveguide region. When this product (f*h).sub.crit, is above a critical value, a dislocation-net is formed and laser performance degrades. At this critical value, the value of f is defined as f.sub.crit. This dislocation-net formation limits the range of the QW compositions and the wavelength available for a given type of laser. Narrowing of QWs has not been shown to be a desirable method to solve these problems, since this approach limits the gain obtainable without saturation effects. In order to achieve gain sufficient to overcome losses, the Multi-Quantum Well (MQW) design may be preferable for many cases.
In the conventional version of separate confinement heterostructure multi-quantum well (SCH MQW) laser structure, the QWs are located in the central part of the waveguide at a distance of 10-20 nanometers (nm) from each other. In this case, the value f.sub.crit is determined by the sum of the QW thicknesses. The strain from each of the QWs adds together, thereby limiting the QW compositions considerably. For example, in the case of InGaAs MQWs grown on a GaAs substrate, the longer wavelength limit is about 1.1 .mu.m and, for InGaAs MQWs grown on an InP substrate, this limit is about 2 .mu.m.
Recent development of semiconductor diode lasers has demonstrated that the thickness of the waveguide region in single SCH-QW and SCH-MQW lasers may be increased to about 1 .mu.m. As described in U.S. patent application Ser. No. 08/757883, filed Nov. 27, 1996 now U.S. Pat. No. 5,818,860, and entitled HIGH POWER SEMICONDUCTOR LASER, which is incorporated herein by reference, high output diode lasers having a waveguide region of 0.7 to 1.3 .mu.m are shown to give satisfactory performance. Also, recent developments show that lasing may occur at wavelengths exceeding 2 .mu.m by employing a MQW structure that is based on AlGaAsSb/InGaAsSb. Semiconductor lasers operating in the mid-infrared wavelength range and formed on binary substrates, such as GaSb or InAs, may use quaternary compounds such as GaInAsSb for the active region since these compounds allow lattice matching of the QW with the substrate under controlled strain. GaInAsSb includes a set of compounds with a lattice constant close to that of the GaSb and InAs, and having a bandgap corresponding to the wavelength range of 2-4 .mu.m.
However, in multi-QW structures of the prior art, the quantum wells were not separated from each other or the cladding layers by a portion of the waveguide region having a thickness much greater than that of the thickness of the QW. This structure design attempts to reduce carrier leakage from the QWs, but at the expense of increased optical losses, since more than 90% of the lasing mode propagates in cladding layers with high free-carrier concentration.
Also, QW lasers with operating ranges of greater than 2.2 .mu.m may be developed in a semiconductor laser system of AlGaAsSb/InGaAsSb with compositions of InGaAsSb QWs close to that of the lattice-matched compounds. These lasers have strain of less than 1% in QW. However, InGaAsSb quaternary compositions of these compounds are close to a miscibility gap. As a result, serious problems are encountered in growing high crystal quality lattice matched InGaAsSb compositions with In contents of more than 20%, which is needed for laser operation at wavelengths longer than 2.2-.mu.m.